1. Field of the Invention
This invention concerns grain oriented ceramics and a production process thereof, as well as an anisotropically-shaped powder A and a production process thereof. More in particular, it relates to grain oriented ceramics suitable as piezoelectric materials for use in the following devices: various types of sensors (e.g. acceleration sensors, piezoelectric sensors, ultrasonic sensors, electrical field sensors, temperature sensors, gas sensors, and knocking sensors); energy conversion devices (e.g. piezoelectric transducers); low-loss actuators or low-loss resonators (e.g. piezoelectric actuators, ultrasonic motors and resonators); capacitors; bimorph piezoelectric devices; vibration pick-up devices; piezoelectric microphones; piezoelectric ignition devices, sonars; piezoelectric buzzers; piezoelectric loud speakers; oscillators; and filters, or suitable as thermoelectric conversion materials or ionic conduction materials, and a production process of such grain oriented ceramics, as well as an anisotropically-shaped powder A suitable to produce such grain oriented ceramics and a production process of such a powder.
2. Description of the Related Art
A piezoelectric material is a material having a piezoelectric effect and is classified, depending on the forms, into single crystals, ceramics, thin films, polymers and composite materials. Among the piezoelectric materials described above, piezoelectric ceramics have been applied generally in the field of electronics or mechatronics since they have high performance and a high degree of flexibility in shape, and since it is easy to design materials for them.
Piezoelectric ceramics are poled polycrystals formed by so-called poling process, that is, by applying a direct current to ferroelectric ceramics and aligning the direction of polarization of the ferroelectric material in a predetermined direction. In order to align spontaneous polarization in a predetermined direction by a poling process in piezoelectric ceramics, a crystal structure of pseudo-isotropic perovskite is advantageous, because spontaneous polarization may be directed three-dimensionally in that structure. Therefore, most of piezoelectric ceramics in practical use are pseudo-isotropic perovskite type (regular perovskite type) ferroelectric ceramics.
Known isotropic perovskite type ferroelectric ceramics may include Pb(Zr.Ti)O3 (hereinafter referred to as “PZT”), PZT ternary systems formed by adding lead-based complex perovskites to PZT, BaTiO3, and Bi0.5Na0.5TiO3 (hereinafter referred to as “BNT”).
Among them, the lead-based piezoelectric ceramics typically represented by PZT have piezoelectric properties better than other piezoelectric ceramics and are predominant among piezoelectric ceramics in practical use at present. However, since they contain lead oxide (PbO) of high vapor pressure, they might put a large load on the environment. Therefore, low-lead or lead-free piezoelectric ceramics having piezoelectric properties equivalent to those of PZT have been demanded.
On the other hand, BaTiO3 ceramics have piezoelectric properties superior to other lead-free piezoelectric materials, and are utilized in sonars, for example. Further, it has been known that some solid solutions of BaTiO3 and other lead-free series perovskite compounds (BNT, for example) also show piezoelectric properties superior to others. Nevertheless, the piezoelectric properties of such lead-free piezoelectric ceramics is inferior to those of PZT.
In view of the above, for solving such problems, various proposals have been made so far. For example, Japanese Patent Application Unexamined Publication No. Hei 11-180769 discloses a piezoelectric ceramics material having a fundamental constitution of (1−x) BNT-BaTiO3 (where x=0.06 to 0.12), and containing 0.5 to 1.5% by weight of rare earth element oxides (for example, La2O3, Y2O3 and Yb2O3).
Further, Japanese Patent Application Unexamined Publication No. 2000-272962 discloses a piezoelectric ceramics composition represented by the general formula: {Bi0.5(Na1−xKx)0.5}TiO3 (where 0.2<x≦0.3) and a piezoelectric ceramics composition containing 2 wt. % or less of additives (Fe2O3, Cr2O3, MnO2, NiO and Nb2O3, for example).
Further, Japanese Patent Application Unexamined Publication No. 2000-281443 discloses a piezoelectric ceramic composition which is mainly constituted of a tungsten bronze type complex oxide represented by the general formula: xNaNbO3-yBaNb2O6-zBiNb3O9 (where x+y+z=1, (x, y, z) are within a predetermined region of a three-component phase diagram), and which contains Bi at a ratio of 3 to 6 wt % with respect to the entire weight where it is converted to metal.
Further, Japanese Patent Application Unexamined Publication No. 2000-313664 filed by the present applicant discloses an alkali metal-containing niobate series piezoelectric ceramics composition formed by adding a compound containing one or more of elements selected from Cu, Li and Ta to a solid solution represented by the general formula: K1−xNaxNbO3. (where x=0 to 0.8).
Further, Japanese Patent Application Unexamined Publication No. Hei 11-60333 filed by the present applicant discloses piezoelectric ceramics which is constituted of perovskite type ceramics containing rhombohedral crystals as the end phase (for example, perovskite type ceramics in which Bi0.5K0.5TiO3, BaTiO3, NaNbO3 or the like is solid solubilized to BNT), and which has a degree of orientation of 30% or more at the pseudo-cubic {100} plane according to the Lotgering's method.
Further, the above-mentioned publication discloses a process of producing piezoelectric ceramics which includes the following steps: a step of mixing a host material A (Ba4Ti3O2), any one of a guest material B (an equiaxial powder of Bi0.5(Na0.85K0.15)0.5TiO3) and a material Q, and a guest material C (a Bi2O3 powder, an Na2CO3 powder, a K2CO3 powder and a TiO2 powder), where the host material A has a platelike shape and is composed of a layered perovskite type compound, the material B has an isotropic perovskite type structure, the material Q is capable of forming the guest material B, and the guest material C converts the host material A into an isotropic perovskite type compound; a step of molding the mixture such that the host material A is oriented; and a step of sintering the green body by heating.
It has been known that when a certain kind of additive is added to a lead-free isotropic perovskite compound such as BaTiO3 or BNT, sinterability and piezoelectric properties are improved. This is also observed in isotropic perovskite type compounds typically represented by a perovskite-type alkali niobate compound (K1−xNaxNbO3) in which the main ingredient of the A site element is K and/or Na and the main ingredient of the B site element is Nb, Sb and/or Ta (hereinafter referred to as a “first perovskite-type alkali-pentavalent metal oxide compound”), and sinterability and piezoelectric properties can be improved by optimizing the kind and the addition amount of the additives.
However, if the first perovskite-type alkali-pentavalent metal oxide compound is produced by an ordinary ceramics-production process in which simple compounds containing ingredient elements are used as starting materials to be calcinated, molded and sintered, each grain in the obtained sintered body is oriented at random. In this case, the resultant sintered body is deficient in piezoelectric properties accordingly, although a perovskite-type alkali-pentavalent metal oxide compound has excellent piezoelectric properties by nature.
Generally, it has been known that the piezoelectric properties of the isotropic perovskite type compounds are different depending on the direction of the crystallographic axis. Accordingly, when the crystallographic axis of high piezoelectric properties can be oriented in a predetermined direction, anisotropy of the piezoelectric properties can be utilized to the utmost degree and it can be expected to make the characteristics of the piezoelectric ceramics higher. Actually, it has been known that some single crystals composed of lead-free series ferroelectric materials show excellent piezoelectric properties.
However, single crystals involve a problem of high production cost. Further, single crystals of a solid solution of complicate composition such as a first perovskite-type alkali-pentavalent metal oxide compound tends to cause deviation of composition during production and is not suitable as practical materials. Further, since single crystals are poor in fracture toughness, they are difficult to be used under high stress, resulting in a limited range of applications.
On the other hand, platelike powder of a layered perovskite type compound (host material A) functions as a reactive template for forming an isotropic perovskite type compound as described in Japanese Patent Patent Application Unexamined Publication No. Hei 11-60333. Therefore, when the host material A is oriented in the green body and reacted with the guest material C, grain oriented ceramics in which a specific crystal plane is oriented at a high degree of orientation can be produced easily and at a reduced cost even from an isotropic perovskite type compound with less anisotropy of crystal lattice.
However, there have been no examples of applying the method described above to the first perovskite-type alkali-pentavalent metal oxide compound. Further, by this method, the isotropic perovskite compound (general formula: ABO3) is formed by reaction between the host material A and the guest material C, and the A-site element (Bi) contained in the host material A (Bi4Ti3O12) always remains in the composition of the resultant grain oriented ceramics. Therefore, this method cannot sometimes attain the most desired composition and the characteristics as the piezoelectric material may possibly be damaged by the A-site element contained inevitably.